1. Field
The disclosed subject matter relates to a semiconductor light emitting device for emitting light on the side of a substrate, and a method for manufacturing the semiconductor light emitting device.
2. Description of the Related Art
There are at least two types of semiconductor light emitting devices. One is a semiconductor light emitting device which emits light from the side of a semiconductor layer laminated on a substrate, and the other is a reflective type of light emitting device (hereinafter referred to as “flip chip”) which emits light on the side of the substrate.
The light emitting device for emitting light from the side of the semiconductor layer uses a translucent electrode as an electrode on the semiconductor layer. Light from a luminescent layer is emitted to the outside through the translucent electrode. According to this structure, the transmittance of the translucent electrode is 70 to 80% at the maximum, and hence light output loss is large.
A frame, a stem, a heat sink, a wiring board, and the like are bonded on the substrate, but the thermal conductivity of the substrate is not high. Taking a case of a sapphire substrate, for example, the thermal conductivity is approximately 40 W/(m·K). Accordingly, there is a problem that when a large current is applied for high power output, heat generation causes reduction in the performance of the device (and device module), causes acceleration of degradation thereof, causes breakage thereof, and the like.
The flip chip, on the other hand, uses a substrate which is transparent with respect to emitted light, and includes a material with high reflectivity (such as Ag) as a p-type ohmic electrode on a semiconductor layer. According to this structure, light is emitted from a luminescent layer, and the portion of that light that is emitted to the side of the substrate passes through the transparent substrate. Light emitted to the side of the p-type ohmic electrode is reflected by the electrode and is emitted from the surface of the substrate. A sapphire substrate, for example, hardly has absorption loss when the emitted light is blue light, so that the sapphire substrate is applicable to the flip chip. The side of the semiconductor layer is bonded to a frame, a stem, a submount, a heat sink, a wiring board, and the like. Thus, the flip chip is superior in point of heat dissipation, so that it is possible to apply a large current for high power output.
Such a flip chip is described in Japanese Patent Laid-Open Publication No. Hei 11-330559, which discloses a flip chip in which an end face of a device is an inclined surface and an n-type electrode is disposed on the inclined surface, in order to reflect part of light that is emitted from a luminescent layer in the direction of the end face of the device towards the direction of the surface of the substrate. Accordingly, not only light that is emitted from the luminescent layer in the direction of the surface of the substrate, but also part of the light that is emitted in the direction of the end face can be emitted from the surface of the substrate, so that emission intensity is increased.
Japanese Patent Laid-Open Publication No. 2002-353504 discloses the structure of a flip chip in which a semiconductor layer takes the shape of a mesa, and a mesa wall is covered with a dielectric with high reflectivity in order to reflect light emitted in the direction of an end face towards the direction of the surface of a substrate.
To reflect light in a desired direction by an inclined end face, it is helpful to precisely form the inclination angle of the end face of a device into a desired angle. In Japanese Patent Laid-Open Publication No. Hei 11-330559, the end face of the semiconductor layer is formed into a desired inclination angle by etching with the use of a resist mask, the end face of which is formed into an inclined shape. The publication, however, does not describe in detail how to incline the end face of the resist mask to the desired angle. Neither of the above-noted references describes a method for controlling the angle of the mesa wall in detail. A method in which a photomask is slightly floated from the resist layer during the exposure of the resist layer is generally known as a method for forming a resist mask having an inclined end face. According to this method, since light at a pattern edge of the photomask goes out of focus, the exposure intensity of a resist in the direction of depth is inclined. Thus, an end portion of the resist takes an inclined shape after development.
In the structure described in Japanese Patent Laid-Open Publication No. Hei 11-330559, the n-type electrode is disposed on the inclined surface of the end face of the device. Since contact between the luminescent layer and the n-type electrode causes short circuiting, the n-type electrode on the inclined surface has to be disposed at a distance away from the end face of the luminescent layer. To increase the amount of light reflected by the n-type electrode, on the other hand, it is helpful to cover the inclined surface of the n-type semiconductor layer having a thickness of several μm with the n-type electrode as widely as possible. To satisfy both of them at the same time, it is desired to bring an end portion of the n-type electrode near to the end face of the luminescent layer to a distance of 1 μm or less. It is difficult, however, to make the distance between the n-type electrode and the end face of the luminescent layer 5 μm or less by a photolithographic technology using a mask aligner, which is generally used in the manufacture of the light emitting device. Using a stepper makes it possible to make the distance 1 μm or less, but manufacturing costs increase. When the distance is 1 μm or less, there is a possibility that dust and a burr occurring in a process after that cause the short circuiting. When difference in a lattice constant between the substrate and the semiconductor layer is large (for example, a combination of a sapphire C-plane substrate and a gallium nitride semiconductor layer), or difference in a thermal expansion coefficient is large, warpage occurs in the substrate. Thus, it is difficult to transfer a mask pattern with high precision of 1 μm or less even if a stepper is used. Furthermore, depending on a material of the electrode, electromigration during use or electrochemical migration may cause the short circuiting, when the end portion of the n-type electrode is brought near to the end face of the luminescent layer to a distance of 1 μm or less. As described above, it is difficult to efficiently reflect light emitted towards the direction of the end face by the n-type electrode on the inclined surface, and maintain electric characteristics.
In the structure described in Japanese Patent Laid-Open Publication No. 2002-353504, on the other hand, since the dielectric with high reflectivity covers the mesa wall, the short circuiting does not occur between the end face of the luminescent layer and the n-type electrode. The reflectivity of the dielectric, however, depends on the incident angle and wavelength of light. Thus, it is difficult to reflect light emitted towards the direction of the end face, the incident angle and wavelength of which expand, with high efficiency like a metal electrode.
To reflect light to a desired direction by the inclined end face, as described in both of the above-referenced publications, it is helpful to precisely form the inclination angle of the end face of the device into a desired angle. The publications do not describe such a method. The method in which the photomask is floated from the resist layer during exposure is easy and convenient. If the offset position (distance that is to be floated from the resist layer) of the photomask is misaligned, a blurry region of light largely varies, and hence there is a problem that an inclination width and the inclination angle largely vary. Since the amount of exposure in the blurry region of light increases and decreases in accordance with the length of exposure time, effective exposure distance in a resist film varies, so that the depth of the formed inclined surface varies. The depth of the inclined surface also varies in accordance with the thickness of the resist. Therefore, it was difficult to form the inclined surface which can precisely reflect light to a desired direction.